The present invention relates generally to a system for controlling an automatic transmission based on self-learning and more particularly, to the system to be arranged in a motor vehicle having a skid reducer for restraining/preventing an acceleration skid.
A conventional system for controlling an automatic transmission based on self-learning is disclosed, for example, in JP-A 63-92863. This system is constructed to detect a shift time from output of a shift command signal by shift command means to completion of shift, and provide to hydraulic pressure command means a correction signal for correcting a hydraulic pressure so that the shift time approaches a reference shift time as previously established.
Recently, some motor vehicles have a skid control device for restraining/preventing an acceleration skid of driving wheels as disclosed in JP-A 3-79853. The skid control device comprises two throttle valves for adjusting an intake air amount of an engine, i.e. a first throttle valve cooperating with an accelerator pedal and a second throttle valve as motor-driven, a sensor for detecting a revolution of non-driving wheels, and a sensor for detecting a revolution of driving wheels. The skid control device is constructed so that when detecting a skid of the driving wheels, the second throttle valve is closed to decrease output torque of the engine, restraining/preventing a skid of the driving wheels.
As for the conventional control system for an automatic transmission, however, due to absence of considering a simultaneous use of the skid control device in the motor vehicle, the following problem arises when arranging the above control system in the motor vehicle having the skid control device:
As described above, when detecting a skid of the driving wheels, the skid control device closes the second throttle valve to decrease output torque of the engine. At this time, a basic hydraulic pressure value should be varied in accordance with this change in output torque. However, since the basic hydraulic pressure value is determined by an opening degree of the first throttle valve, i.e. based on output torque during normal operation with no skid control, self-learning is erroneously carried out, i.e. an increase and decrease in a correction value is not carried out appropriately. Thus, upon subsequent shift, the optimum hydraulic pressure value determined by hydraulic pressure determination means does not correspond to an appropriate value. If the hydraulic pressure value does not correspond to the appropriate value, it is possible to aggravate shift shock or have a bad influence on the durability of friction engagement elements.
Moreover, even if the hydraulic pressure determination means determine a basic hydraulic pressure value based on a smaller one of opening degrees of the first and second throttle valves, an intake system of the engine is affected by the other throttle valve, failing to detect accurate output torque of the engine. Thus, the optimum hydraulic pressure value is established to a value near the appropriate value but with some deviation, and cannot be established to the appropriate value corresponding to output torque of the engine. Therefore, when carrying out self-learning for determining a correction value according to such inappropriate shift, self-learning is erroneously carried out, producing the above inconvenience.
It is, therefore, an object of the present invention to provide a control system for an automatic transmission which ensures accurate self-learning in a motor vehicle having a skid control device.